1. Field of the Invention
This invention generally relates to phase-locked loop (PLL) circuitry and, more particularly, to a wide dynamic range charge pump for use in a PLL.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a conventional charge pump and loop filter (prior art). The charge pump 100 and loop filter 102 are interposed between a phase detector (PHD) or phase/frequency detector (PFD) 104 and a voltage controlled oscillator (VCO), not shown. For simplicity, the common mode feedback (CMF) circuitry is not depicted. The advantage of this circuit is its simplicity. In less critical applications, such as those in which there is no restriction on the maximum loop bandwidth and/or jitter transfer peaking, the circuit is easily implemented. The disadvantage is that the circuit does not permit the independent adjustment of loop bandwidth and jitter transfer peaking. That is, the loop bandwidth and the jitter transfer peaking can not be optimized independently.
FIG. 2 is a schematic diagram of a charge pump and loop filter that circumvents the problems associated with the circuit of FIG. 1 (prior art). Since there are 2 separate current outputs (Ia, and Ib), the jitter transfer peaking and the loop bandwidth can be adjusted independently. The current, Ia, controls the jitter transfer peaking only, and the current, Ib, controls the loop bandwidth as well as the peaking. However, after the loop bandwidth is set, the peaking can be re-adjusted by Ia without changing the set bandwidth.
One problem observed in the circuit of FIG. 2 is its inability to accommodate large values of Ib, which is limited by the driving capacity of the unity gain buffers 200 and 202. The difficulty arises from the fact that the unity gain buffer is an operational amplifier (op amp) in the voltage-follower configuration. There is really no better unity gain alternative. In the latest technologies, supply voltages are as low as 1.2V. The output of the op amp needs to swing as large as its input, and still maintain a high enough open loop gain so that the close loop gain will be no less than 0.85 worst-case (a reasonable design target). These requirements restrict the driving capability of the output stage. When Ib must be larger than the op amp can drive, the loop bandwidth can no longer be controlled properly.
It would be advantageous if the charge pump of FIG. 2 could be modified to provide a more symmetric, wider dynamic range at its outputs, while increasing the loop bandwidth adjustment range.